Method for production of natural L-cysteine by fermentation

A method for the targeted control of fractions of L-cysteine, L-cystine and thiazolidine in a total cysteine in a fermentation medium during a preparation by fermentation of natural L-cysteine in a production fermenter with a nominal volume ≧1 m3, said method comprising culturing a microorganism strain from the Enterobacteriaece family in the fermentation medium, wherein the microorganism strain both has a modified serine O-acetyl transferase which, compared to a corresponding wild type enzyme, has a feedback inhibition by L-cysteine reduced by at least a factor of 2, and also has, as a result of overexpression of an efflux gene, a cysteine export from a cell that is increased by at least a factor of 2 compared to a wild type cell, wherein the iron concentration is at a maximum of 8 mg/l and a minimum of 0.6 mg/l and is achieved by increasing the L-cysteine fraction of the total cysteine to at least 65% in the fermentation medium.